Question

Which of the following is reducing sugar?
a.) Galactose
b.) Gluconic acid
c.) β–methyl galactoside
d.) sucrose

Answer 1

Hint: The reducing sugar or monosaccharides have free aldehyde or ketone groups in their structures. Aldehyde or ketone groups can reduce $$C{u}^{2+}$$ to $$Cu$$. Therefore, they are called reducing sugars. Glucose, fructose, mannose, galactose are hexose monosaccharides good examples for reducing sugar.
Complete answer:
Reducing sugar means they have a capability to donate electrons to other chemicals.
Reducing sugars themselves they oxidize and reduce the other chemicals that is why they are called as reducing sugars.
Reducing sugars are always monosaccharides. Means reducing sugars always exist as single molecules.
Coming to given options, option B, Gluconic acid. In the name it is mentioned that it has a carboxyl group. But reducing sugar should have a free aldehyde or ketone groups. So, option b is wrong.
Coming to option C, β–methyl galactoside does not have a free aldehyde or ketone group in its structure. So, option C is wrong.
Coming to option D, Sucrose. It is not a monosaccharide. It is a disaccharide containing glucose and fructose in its structure.
So, the correct option is A. Galactose is a monosaccharide containing free aldehyde or ketone groups and converts $$C{u}^{2+}$$ to $$Cu$$.
Note: Don’t be confused with the words monosaccharide and disaccharide.
Monosaccharide: They are also called simple sugars. Example: Glucose, Galactose, Fructose etc.
Disaccharide: It is the sugar formed when two monosaccharides are joined by glycosidic linkage. Example: Sucrose, lactose, maltose etc.